Re: [PATCHv7] rtc: Add support for Intersil ISL12057 I2C RTC chip

[arno@natisbad.org (Arnaud Ebalard)]

Hi Andrew,

Andrew Morton <akpm@linux-foundation.org> writes:

> On Thu, 19 Dec 2013 22:53:16 +0100 Arnaud Ebalard <arno@natisbad.org> wrote:
>
>> +static int isl12057_i2c_validate_chip(struct regmap *regmap)
>> +{
>> +	u8 regs[ISL12057_MEM_MAP_LEN];
>> +	u8 mask[ISL12057_MEM_MAP_LEN] = { 0x80, 0x80, 0x80, 0xf8,
>> +					  0xc0, 0x60, 0x00, 0x00,
>> +					  0x00, 0x00, 0x00, 0x00,
>> +					  0x00, 0x00, 0x60, 0x7c };
>> +	int ret, i;
>> +
>> +	ret = regmap_bulk_read(regmap, 0, regs, ISL12057_MEM_MAP_LEN);
>> +	if (ret)
>> +		return ret;
>> +
>> +	for (i = 0; i < ISL12057_MEM_MAP_LEN; ++i) {
>> +		if (regs[i] & mask[i])	/* check if bits are cleared */
>> +			return -ENODEV;
>> +	}
>> +
>> +	return 0;
>> +}
>
> When I build this for x86_64, mask[] is assembled on-stack at runtime:
>
> 	leaq	-48(%rbp), %r14	#, tmp130
> 	xorl	%esi, %esi	#
> 	movl	$16, %ecx	#,
> 	movq	%rax, %rdi	# regmap,
> 	incq	.LPBX1+552(%rip)	# *.LPBX1
> 	movb	$-128, -64(%rbp)	#, mask
> 	movq	%r14, %rdx	# tmp130,
> 	movb	$-128, -63(%rbp)	#, mask
> 	movb	$-128, -62(%rbp)	#, mask
> 	movb	$-8, -61(%rbp)	#, mask
> 	movb	$-64, -60(%rbp)	#, mask
> 	movb	$96, -59(%rbp)	#, mask
> 	movb	$0, -58(%rbp)	#, mask
> 	movb	$0, -57(%rbp)	#, mask
> 	movb	$0, -56(%rbp)	#, mask
> 	movb	$0, -55(%rbp)	#, mask
> 	movb	$0, -54(%rbp)	#, mask
> 	movb	$0, -53(%rbp)	#, mask
> 	movb	$0, -52(%rbp)	#, mask
> 	movb	$0, -51(%rbp)	#, mask
> 	movb	$96, -50(%rbp)	#, mask
> 	movb	$124, -49(%rbp)	#, mask
> 	call	regmap_bulk_read	#
>
> If we make `mask' static const, that all gets done at compile time instead:
>
> 	leaq	-48(%rbp), %r14	#, tmp134
> 	xorl	%esi, %esi	#
> 	movl	$16, %ecx	#,
> 	movq	%rax, %rdi	# regmap,
> 	incq	.LPBX1+552(%rip)	# *.LPBX1
> 	movq	%r14, %rdx	# tmp134,
> 	call	regmap_bulk_read	#
>
> much nicer.

Damned. That's a good argument for a v8 ;-) Give me some minutes to
assemble one.

Cheers,

a+